Glaucoma is an ocular disorder associated with elevated intraocular pressures which are too high for normal function and may result in irreversible loss of visual function. If untreated, glaucoma may eventually lead to blindness. Ocular hypertension, i.e., the condition of elevated intraocular pressure without optic nerve head damage or characteristic glaucomatous visual field defects, is now believed by many ophthalmologists to represent the earliest phase of glaucoma.
Many of the drugs formerly used to treat glaucoma proved not entirely satisfactory. Indeed, few advances were made in the treatment of glaucoma since pilocarpine and physostigmine were introduced. Only recently have clinicians noted that many .beta.-adrenergic blocking agents are effective in reducing intraocular pressure. Many of these agents, however, also have other characteristics, e.g. membrane stabilizing activity, that are not acceptable for chronic ocular use. (S)-1-tert-Butylamino[(4-morpholino-1,2,5-thiadiazol-3-yl)oxy]-2-propanol, a .beta.-adrenergic blocking agent, was found to reduce intraocular pressure and to be devoid of many unwanted side effects associated with pilocarpine and, in addition, to possess advantages over many other .beta.-adrenergic blocking agents, e.g. to be devoid of local anesthetic properties, to have a long duration of activity, and to display minimal tolerance.
Although pilocarpine, physostigmine and the .beta.-blocking agents mentioned above reduce intraocular pressure, none of these drugs manifests its action by inhibiting the enzyme carbonic anhydrase and, thereby, impeding the contribution to aqueous humor formation made by the carbonic anhydrase pathway.
Agents referred to as carbonic anhydrase inhibitors block or impede this inflow pathway by inhibiting the enzyme, carbonic anhydrase. While such carbonic anhydrase inhibitors are now used to treat intraocular pressure by oral, intravenous or other systemic routes, they thereby have the distinct disadvantage of inhibiting carbonic anhydrase throughout the entire body. Such a gross disruption of a basic enzyme system is justified only during an acute attack of alarmingly elevated intraocular pressure, or when no other agent is effective. Despite the desirability of directing the carbonic anhydrase inhibitor only to the desired opthalmic target tissue, no topically effective carbonic anhydrase inhibitors are available for clinical use.
However, topically effective carbonic anhydrase inhibitors are reported in U.S. Pat. Nos. 4,386,098; 4,416,890; and 4,426,388. The compounds reported therein are 5 (and 6)-hydroxy-2-benzothiazolesulfonamides and acyl esters thereof. Benzothiophene-2-sulfonamides, benzenesulfonylthiophene-2-sulfonamides, and thieno[2,3-b]thiopyran-2-sulfonamides are also reported to be carbonic anhydrase inhibitors topically effective in reducing intraocular pressure in U.S. Pat. Nos. 4,668,697; 4,585,787; and 4,797,413, respectively.
U.S. Pat. No. 4,619,939 discloses a process and composition for reducing intraocular pressure and reducing aqueous humor formation by applying topically to the cornea an effective amount of an aqueous solution of a carbonic anhydrase inhibitor having the following properties:
a. sufficiently soluble in water to form at least a 3 mM solution at pH 8.2 or a pKa of not greater than 7.3;
b. ether partition coefficient of at least 1.0;
c. chloroform partition coefficient of at least 0.01;
d. dissociation constant against carbonic anhydrase of not more than 3.times.10.sup.-8 molar;
e. first order rate constant for penetration of the sulfonamide through a living rabbit cornea of at least 0.005 hr.sup.-1 ;
f. not injurious to the cornea; and
g. stable in aqueous solution and in contact with the cornea.